Mobile communication devices, such as cellular telephones, have gained widespread use in virtually all metropolitan areas of the world, and a significant amount of speech communication is now performed using mobile telephones. However, due to the mobile nature of these devices, they are inherently vulnerable to use in a wide variety of acoustic environments, some of which may be noisy. Environmental noise may cause problems whether it occurs at the receiving end of a communication, the transmitting end, or a combination (to whatever extent) of the two.
It is known that background noise causes speech intelligibility to be degraded, because speech intelligibility decreases with decreasing signal to noise ratio SNR, and efforts have been made in recent years to improve speech intelligibility in adverse noise conditions. For example, U.S. Pat. No. 6,741,873 describes a mobile communication device in which a background noise level is determined at a microphone and a threshold is established. If the threshold is exceeded, it is determined to be likely that voice energy is being received at the microphone. Thus, if the input signal exceeds the threshold, the mobile communication device transmits the input signal, and the threshold varies dependent on the level of background noise.
However, this arrangement does not necessarily improve speech intelligibility in adverse noise conditions; it simply attempts to reduce the significance of the background noise relative to the speech signal according to the listener's perception, thereby increasing the likelihood of the speech being more intelligible to the listener. However, it is highly desirable to actually improve speech intelligibility in a mobile communication device so as to enhance its performance in a variety of acoustic environments.